1. Field of the Invention
This invention relates to a novel polymer capable of producing a resist pattern having improved resolution, developability and resist pattern profiling in fine processing, having improved shelf stability and useful as a base polymer for ULSI resist material, as well as a negative radiation-sensitive resist composition comprising the polymer as a base polymer.
2. Prior Art
Functional polymers are widely used as base polymers for resist materials which are utilized in high resolution lithography in the manufacture of LSI. A demand for higher resolution and developability is imposed on the resist material to cope with the recent advances in fine processing technology and the increasing degree of integration.
To meet the demand, as long as negative radiation-sensitive resist compositions are concerned, active research works have focused on a three component system comprising an alkali-soluble base polymer, a photo-acid generator capable of generating an acid upon exposure to radiation, and a crosslinking agent capable of causing crosslinking of the base polymer molecules by interacting with the acid. Typical alkali-soluble base polymers used heretofore are novolak resins, polyvinyl phenol derivatives, and N-(p-Hydroxyphenyl)maleimide polymers. Typical photo-acid generators used heretofore are 2,4,6-Tris(trihalomethyl)-s-triazine, 2,4,6-Tris(2,3-dibromopropyl)isocyanurate, and onium salts. Typical crosslinking agents used heretofore are Hexaalkylolmelaminehexamethylethers, Tetraalkoxymethylureas, Dialkoxymethylglycolureas, and dialkoxymethylpropylene ureas.
Resist compositions of the three component system, however, have the following problem. Crosslinking agents, typically Hexamethylolmelaminehexamethylether and Tetramethoxymethylurea are generally difficult to purify and inevitably contain noticeable amounts of impurities in the form of di- to penta-methoxymethylated derivatives. For example, Hexamethylolmelaminehexamethylether contains such impurities as 2,4-(N-Methoxymethyl)amino-6-amino-1,3,5-triazine, 2,4,6-(N-Methoxymethyl)amino-1,3,5-triazine, 2,4-(N-Methoxymethyl)amino-6-(N,N-dimethoxymethyl)amino-1,3,5-triazin and 2,4-(N,N-Dimethoxymethyl)amino-6-(N-methoxymethyl)amino-1,3,5-triazine. If any one of these agents is used alone as a compound having a substituent capable of crosslinking with the aid of an acid, the concomitant impurities can cause striation by exacerbating the compatibility with the base polymer upon formulation of a resist composition, reduce contrast upon fine processing, allow unexposed portions of resist film to be left after development, degrade a resist pattern profile, and detract from the shelf stability of the resist composition itself.
There is a need to have a resist composition which can overcome the above-mentioned problems and has higher resolution and developability.